1. Field of the Invention
The present invention relates to sensors for the detection of urea.
2. Description of Related Art
Continuous monitoring of urea has been accomplished by a number of electrochemical methods (Goldfinch, M. J.; Lowe, C. R. Anal. Biochem. 138: 430-436 (1983); Luo, S.; Walt, D. R. Anal. Chem. 61: 1069-1072 (1989)). Sensors utilizing such methods are created by immobilizing the enzyme urease onto the surface of an electrode. The enzymatic hydrolysis of urea produces ammonia and carbon dioxide, which are protonated at physiologic pH to form ammonium and carbonate ions, which increase the electrical conductivity of the solution proximal to the electrode.
Urea can also be monitored using an optical sensor. The detection of analytes by optical sensors usually requires the development of fluorescent transducers which are specific for different analytes. Optical transducers have also been coupled to the detection of urea via the urease driven hydrolysis of urea, with the optical transducer modulated by ammonium or ammonia.
Detection of ammonium requires an ammonium specific ionophore coupled to a chromophore that changes its absorption spectrum upon protonation, and a lipophilic anionic site. As such, sensors based on the detection of ammonium can be expensive and complex.
Detection of ammonia requires a protonated pH sensitive indicator (INDH.sup.+) which changes its absorption or fluorescence spectrum upon deprotonation:
INDH.sup.+ +NH.sub.3 .fwdarw.IND+NH.sub.4.sup.+
There is also a drawback to designing a sensor based on detection of ammonia: namely the rapid protonation of ammonia at physiologic pH. The pK.sub.a of ammonium is 9.3, which is not a pH that supports maximum enzyme activity.
Hydrophobic polymers, optically transparent and permeable to the analyte of interest, are used with optical sensors when the analyte is a vapor or gas and is capable of diffusion into a hydrophobic membrane. A complication arises when hydrophobic polymers are used with certain fluorescent dyes. Sensors for ammonia require a protonated indicator. When combined with a hydrophobic membrane for the detection of ammonia, polyanionic pH indicators, which are the common variety of protonated indicator and the type used in the fluorescent urea sensor described in Rhines and Arnold (Anal. Chim. Acta, 231: 231-235 (1990)), do not produce an activated and protonated fluorophore.
While various indicators for urea are known, many urea sensors exhibit problems with interferences from pH and CO.sub.2 effects, low sensitivity, slow response times and reversibility. From a manufacturing standpoint, it would therefore be desirable to develop an inexpensive sensor capable of detecting urea that has a high sensitivity, fast response time, and is reversible. It would also be advantageous for the sensor to be able to function in conjunction with sensors detecting other analytes.